Image-reading devices having an automatic document feeding function are known in the art. One such image-reading device performs edge detection on image data acquired by an image-reading unit when reading an original sheet, and determines whether a double-feed occurred when feeding the original sheet on the basis of the edge detection results. In order to reduce processing load, this image-reading device performs the edge detection process only in a region spanning a prescribed number of pixels from the top edge of a page worth of image data along the original sheet feeding direction.